They're good in RTS games. Have the expanded map on one screen and the rest on the other. Supreme Commander and World in Conflict support this option I think (pretty sure WiC does).
I have a 13" Macbook (white), and hopefully tomorrow the nice UPS man will drop off my Mini DVI to VGA so I can hook it up to my new and spiffy 19 inch widescreen monitor.
I tried using dual monitors with my 24" iMac a few months ago.
I bought the DVI adapter, but it didn't work. In retrospect, I think I may have somehow broken the DVI cable that I got with the monitor. I just haven't gotten around to replacing it.
At this time, it would be a major chore to hookup the 19" widescreen monitor, along with the 24" iMac. There just isn't enough room in my tiny apartment to rearrange everything.
Yep, I use an external 17" display as my main workspace and the 17" laptop display as my secondary. Hopefully I'll be getting a matrox dual head to go or a usb video adapter so I can power the second external 17" display I have. Might also get 2 22" displays or 2 24" displays.